Rotor blade of a wind turbine

ABSTRACT

A rotor blade of a wind turbine with a raked tip portion is provided. The tip portion is raked, i.e. curved, in a rotor blade plane comprising the tip base chord and a line which is parallel to the pitch axis of the rotor blade. Additionally, the orientation of the chords with reference to the chord at the tip base changes between the tip base and the tip. In other words, the trailing edge of the tip portion is twisted. Optionally, the tip portion is additionally swept out of the rotor blade plane, which is characterized by a cant angle.

FIELD OF INVENTION

The present invention relates to a rotor blade of a wind turbine. Inparticular, the invention relates to means for improving efficiency of awind turbine with such a rotor blade.

BACKGROUND OF THE INVENTION

Wind turbines are a very attractive option for generating electricity. Awind turbine typically comprises one or several rotor blades, which arerotatable mounted to a hub of the wind turbine. A rotor blade in generalcomprises a root portion, by which the rotor blade is connected to thehub. Furthermore, a rotor blade comprises a blade body, which is alsoreferred to as the airfoil-shaped portion of the rotor blade, and a tipportion, which is opposite to the root portion.

The tip portion of a rotor blade encounters conflicting designrequirements which shall be satisfied. On the one side, the tip portionshall unload the aerodynamic loading on the rotor blade and shallprovide a minimum tip vortex to reduce downwash. This requires a designthat reduces the aerodynamic loading progressively along the span lineof the tip portion and thereby lowers local axial induction. On theother hand, the tip portion shall produce as much power as possible andshall thus contribute to the overall purpose of a rotor blade, which isthe improvement of efficiency of a wind turbine comprising this rotorblade. For the tip portion, this requires maintaining an axial inductionas close as possible to the aerodynamic optimum.

Advantageously, these two design requirements are met without inducingflow separation of the airflow which would generate undesirably highlevels of aerodynamic noise.

Current tip portions of rotor blades are typically designed toaccomplish the first design requirement: Aerodynamic unloading with theprovision of small tip vortices. This requirement may, for instance, bemet by so-called winglets, which comprise a curvature of the rotor bladein the section of the tip portion out of a plane of the rotor blade.However, no satisfying solution for a design of a tip portion of a rotorblade, wherein the tip portion satisfactorily meets both designrequirements mentioned above, has been presented yet.

Thus, there exists an urgent need to provide a rotor blade of a windturbine, wherein the rotor blade comprises a tip portion and the tipportion is designed such that efficiency of a wind turbine comprisingsuch a rotor blade is improved.

SUMMARY OF THE INVENTION

This objective is achieved by the independent claims. The dependentclaims describe advantageous developments and modifications of theinvention.

In accordance with the invention there is provided a rotor blade of awind turbine wherein the rotor blade comprises a blade body extendingbetween a root portion of the rotor blade and a tip portion of the rotorblade. The rotor blade comprises a trailing edge and a leading edge. Therotor blade is arranged and prepared for being mounted to a hub of thewind turbine and for being pitched about a pitch axis. A rotor bladeplane is defined by the plane comprising the chord at the tip base and aline which is parallel to the pitch axis, wherein the tip base is thepart of the rotor blade at which the tip portion joins the blade body.Furthermore, the trailing edge of the tip portion has a curved shape asprojected on the rotor blade plane in a way that a trailing edge sweepangle increases from the tip base to the tip of the rotor blade.Additionally, orientation of the chords with reference to the chord atthe tip base changes between the tip base and the tip in a way that achord tilt angle changes between the tip base and the tip.

Figuratively speaking, the trailing edge of the tip portion of the rotorblade comprises a curved or swept shape as projected on the rotor bladeplane and, at the same time, the chords of the tip portion of the rotorblade change their orientation with regard to the tip base chord. Thus,superimposing the curvature of the trailing edge in the rotor bladeplane and the changing orientation of the chords results in a curved andtwisted trailing edge between the tip base and the tip.

Rotor blades with a curved tip portion including a curved trailing edgewith the curvature realized in the rotor blade plane are known to theperson skilled in the art. This invention, however, discloses a tipportion, where the chords and thus the trailing edge as well areadditionally twisted. By the combination of sweep and twist at the tipportion of the rotor blade the two conflicting design requirements whichhave been described above are successfully met. More specifically, thecurved and twisted trailing edge of the tip portion involves both alowering of the local axial induction in one segment of the tip portion,thus reducing the aerodynamic loading, and at the same time maintenanceof the axial induction as close as possible to the aerodynamic optimumon a different segment of the tip portion, thus improving the powergenerating potential of the rotor blade.

Note that in the context of this application, a chord, which always is astraight line and which, for instance, is not curved of twisted itself,may be tilted, in other words inclined, with reference to a referenceline, which may be the chord at the tip base. The result of a pluralityof tilted chords comprising different chord tilt angles is a trailingedge, which is twisted, in other words bent, and which is thus referredto as a twisted trailing edge.

Additionally, note that as, for instance, the rotor blade may be curvedto the suction side or to the pressure side such that the pitch axis andthe chord at the tip base do not intersect and thus cannot define aplane, the rotor blade plane is defined by any line which is parallel tothe pitch axis and which can validly define a plane. This is inparticular the case for a line being parallel to the pitch axis whichintersects the chord at the tip base.

An advantage of the disclosed rotor blade is an increased powerproduction capability of the wind turbine where the rotor blade isattached to due to higher pressure recoveries for the rotor blade nearthe tip portion. Additionally, acoustic noise may be reduced due to anadvantageous angle between the trailing edge and the streamline of theairflow which flows from the leading edge across the rotor blade.

Another advantage of the inventive tip portion is a reduction, or even aprevention, of flow separation, thus leading to a promotion of tipvortices which are small compared to tip vortices of a similar,conventional rotor blade.

In other words, the tip portion may be described as being raked in anedgewise direction of the rotor blade.

A span line of the rotor blade is referred to as a longitudinal axis ofthe rotor blade extending from the root to the tip. Compared to thepitch axis, which always is a straight line, the span line follows theactual shape of the rotor blade. This means that if e.g. the blade bodycomprises a slight aft-swept, the span line is slightly aft-swept, too.The span line follows the shape of rotor blade in its tip portionlikewise. Thus, a curved tip portion results in a curved span line.

At each radial position along the span line, in particular at eachradial position of the blade body and of the tip portion, a chord can beassigned to the rotor blade. In the context of this application, thechord is defined as being the line connecting the trailing edge and theleading edge in a plane perpendicular to the pitch axis. This plane isalso referred to as the specific cross-section profile or airfoil of therotor blade at a radial position.

The tip base is a plane which is perpendicular to the pitch axis. Thechord which is determined at the tip base is also referred to as the tipbase chord.

In particular, the tip base may be understood as the plane beingperpendicular to the pitch axis at the specific radial position of thepitch axis where the trailing edge starts to describe a concavecurvature and/or the leading edge starts to describe a convex curvature,as viewed from the suction side of the rotor blade.

Furthermore, a reference plane can be assigned to the rotor blade. Thereference plane is defined as the plane comprising the pitch axis andbeing perpendicular to the tip base chord.

The trailing edge sweep angle is defined as the angle between the pitchaxis and the trailing edge and is determined at the tip base. As knownfrom geometry, an angle in general is defined by a first line, a secondline and an intersection of both lines, which is also referred to as thevertex. Thus, precisely speaking, the first side of the trailing edgesweep angle is the tangent of the trailing edge and the second side ofthe trailing edge sweep angle is a line which is parallel to the pitchaxis and which intersects the intersection of the tangent of thetrailing edge and the tip base. This intersection may be referred to asthe vertex.

For any point along the trailing edge of the tip portion, a trailingedge sweep angle may be defined. It is one aspect of the invention thatthe trailing edge sweep angle increases from the tip base to the tip. Inother words, the trailing edge sweep angle measured at the tip isgreater than the trailing edge sweep angle measured at the tip base.

It has to be noted that the expression of the curved shape of theprotection of the trailing edge of the tip portion on the rotor bladeplane comprises a continuously curved shape of the trailing edge, butalso a shape of the trailing edge resembling a polygon. In other words,the trailing edge of the tip portion may partially or fully be steppedor segmented.

The chord tilt angle is defined as the angle between the tip base chordand a specific chord at a specific radial position between the tip baseand the tip. Mathematically speaking, the first side of the chord tiltangle is represented by the specific chord and the second line isrepresented by a line which is parallel to the tip base chord andintersects the specific chord.

In other words, one aspect of the invention is characterized by the factthat the orientation of the chords of the tip portion changes betweenthe tip base and the tip, compared to the chord at the tip base.

The blade body may be substantially straight, meaning that the span lineof the blade body substantially coincides with the pitch axis. However,the curved and twisted tip portion may also be combined with a bladebody which itself comprises a swept or curved shape. Thus, the raked tipportion may be combined with any shape of the blade body of the rotorblade.

A wind turbine, which is also referred to as a wind power plant, is adevice that converts kinetic energy from the wind into electricalenergy.

The tip base is a virtual or imaginary surface or area within the rotorblade for defining the limit of the tip portion. If, as an example, thetip portion is manufactured separately with regard to the blade body,the tip base may be the real and actual area where the tip portion isconnected or attached to the blade body. However, this is notnecessarily the case, as the rotor blade comprising the tip portion mayalso be manufactured unitarily, i.e. as a single piece.

It has to be noted that the trailing edge sweep angle does notnecessarily have to increase along the whole way from the tip base tothe tip. However, there have to be parts or sections of the trailingedge of the tip portion, wherein the trailing edge sweep angleincreases.

In an advantageous embodiment, the trailing edge sweep angle increasesby at least 2 degrees, in particular by at least 5 degrees, from the tipbase to the tip.

In another advantageous embodiment, the trailing edge sweep angleincreases by at most 40 degrees, in particular by at most 30 degrees,from the tip base to the tip.

Note that the given values for the trailing edge sweep angle representadvantageous minimum and maximum values, respectively. These minimum andmaximum trailing edge sweep angles refer to a comparison of the trailingedge sweep angle determined at the tip base with the trailing edge sweepangle determined at the tip. If, for instance, the trailing edge iscurved evenly from the tip base to the tip and the trailing edge sweepangle increases by 2 degrees between the tip base and the tip, thiswould imply that the trailing edge sweep angle determined halfwaybetween the tip base and the tip only increases by 1 degree compared tothe tip base.

In the exemplarily case of a straight blade body, the trailing edge maybe parallel to the pitch axis at the tip base. In this case, thetrailing edge sweep angle is 0 degree at the tip base. Due to the curvedtrailing edge of the tip portion, the trailing edge sweep angle may forexample be 10 degrees at the tip. Then, the trailing edge sweep angle isdescribed as increasing by 10 degrees between the tip base and the tip.If, however, in another example the trailing edge comprises an angle of7 degrees with the pitch axis at the tip base, which may well be thecase of a swept blade body, and the trailing edge sweep angle is 10degrees at the tip, then the trailing edge sweep angle increases by only3 degrees between the tip base and the tip.

In other words, the advantageous minimum and maximum values of 2 degreesand 40 degrees relate to a relative increase of the trailing edge sweepangle from the tip base to the tip.

In another advantageous embodiment, the chord tilt angle varies by atleast 2 degrees, in particular by at least 5 degrees, between the tipbase and the tip.

In another advantageous embodiment, the chord tilt angle varies by atmost 30 degrees, in particular by at most 20 degrees, between the tipbase and the tip.

Similarly as for the trailing edge sweep angle, the given advantageousvalues of minimum and maximum chord tilt angles, respectively, refer toa relative variation of the chord tilt angle between the tip base andthe tip.

It may be advantageous that the chord tilt angle features both sectionswith increase and sections with decrease along the pitch axis betweenthe tip base and the tip. In particular, the tip portion may comprise afirst section, where the trailing edge is twisted towards a suction ofthe blade body, and/or a second section, where the trailing edge istwisted towards a pressure side of the blade body. Suction and pressureside are commonly used expressions in the art of rotor bladeaerodynamics.

In other words, it may be advantageous that the chords are inclined ororientated to one side in the first section, thus exhibiting negativechord tilt angles in the first section, and that they are inclined ororientated to the other side in the second section, thus exhibitingpositive chord tilt angles in the second section. An inclination towardsthe suction side is referred to negative chord angles and an inclinationtowards the pressure side is referred to positive chord angles

The radial position where the chord tilt angles change from a decreasingslope to an increasing slope is also denoted as a toe.

It may be advantageous that the first section is adjacent to the tipbase, while the second section is adjacent to the tip.

Figuratively speaking, coming from the tip base and heading towards thetip, in one embodiment of the invention the chords are firstly inclinedtowards the suction side of the blade body, reach a maximum inclinationto the suction side, which is referred to as the toe, and subsequentlyare increasingly inclined to the pressure side of the blade body untilreaching the tip with a considerable inclination towards the pressureside.

It may be advantageous that the increase of the chord tilt angle in thesecond section is steep compared to its decrease in the first section.

In another advantageous embodiment, the chord length decreases at ahigher rate in the second section compared to the first section.

It has to be found that the combination of a relatively strong decreaseof the chord length in a section close to the tip, combined with arelatively strong increase of the chord tilt angle may provide anexceptionally large improvement of the efficiency of the wind turbineand an exceptionally large reduction of the aerodynamic noise generatedby the rotor blade.

In another advantageous embodiment, the projection of the leading edgeof the tip portion on the rotor blade plane has a curved shape such thata leading edge sweep angle increases from the tip base to the tip.

The leading edge sweep angle is defined as the angle between the pitchaxis and the leading edge and is determined at the tip base. Preciselyspeaking, the first side of the leading edge sweep angle is the tangentof the leading edge, and the second side of the leading edge sweep angleis a line which is parallel to the pitch axis and which intersects theintersection of the tangent of the leading edge and the tip base. Thisintersection may be referred to as the vertex of the leading edge angle.

Advantageously, the leading edge sweep angle increases by at least 10degrees, in particular by at least 20 degrees, from the tip base to thetip.

In another advantageous embodiment, the leading edge sweep angleincreases by at most 80 degrees, in particular by at most 60 degrees,from the tip base to the tip.

As in general the tip portion does not only comprise a trailing edge butalso a leading edge, the tip portion may also be characterized by theleading edge sweep angle. The leading edge sweep angle is definedsimilarly to the trailing edge sweep angle. If the tip portion is curvedtowards the trailing edge of the blade body, the leading edge sweepangle is generally larger than the trailing edge sweep angle. If the tipportion is curved towards the leading edge of the blade body, theleading edge sweep angle is generally smaller than the trailing edgesweep angle.

Advantageously, the tip portion is curved towards the trailing edge ofthe blade body, which is also referred to as an aft-sweep.

A curvature towards the trailing edge of the blade body has theadvantage of an advantageous angle between the streamline of an airflowwhich flows from the leading edge across the rotor blade and thetrailing edge, which may lead to an overall reduction of aerodynamicnoise, i.e. a reduction of the sound pressure level generated by therotor blade. This is due to directivity effects.

Note that the advantageous minimum and maximum values for the leadingedge sweep angles have to be understood similarly as the minimum andmaximum values for the trailing edge sweep angle.

In another advantageous embodiment, the trailing edge of the tipportion, as viewed from the suction side of the rotor blade, is at leastpartially concavely shaped and the leading edge of the tip portion, asviewed from the suction side of the rotor blade, is at least partiallyconvexly shaped.

In particular, the whole trailing edge of the tip portion may beconcavely shaped and/or the whole leading edge of the tip portion may beconvexly shaped.

In another advantageous embodiment, the projection of the trailing edgeof the tip portion on the reference plane has a curved shape such that acant angle increases from the tip base to the tip.

The cant angle is defined as the angle between the trailing edge and theline which is parallel to the pitch axis and intersects the intersectionof the tangent of the trailing edge with the tip base. This intersectionis referred to as the vertex of the cant angle.

Figuratively speaking, the cant angle refers to a sweep of the tipportion out of the rotor blade plane, while the trailing edge sweepangle refers to a sweep of the trailing edge within the rotor bladeplane. The twist of the trailing edge, characterized by the chord tiltangle, differs from the cant of the tip portion, characterized by thecant angle, in that the twist basically relates to a displacement oroff-set of solely the trailing edge, while the tip portion as a wholesubstantially remains in the rotor blade plane, compared to the cant,which basically relates to a displacement of off-set of the tip portionas a whole out of the rotor blade plane.

It may thus be advantageous to provide a tip portion wherein thetrailing edge comprises a sweep within the rotor blade plane,characterized by the trailing edge sweep angle, a sweep out of the rotorblade plane, characterized by the cant angle, and a twist of thetrailing edge, characterized by the chord tilt angle. By combining allthree features, an optimal design of the tip portion can be realized.

The sweep of the tip portion which is characterized by the cant angle isalso referred to as a flapwise rake of the rotor blade. Thus, in otherwords, an edgewise rake and a flapwise rake may be superimposed toachieve an optimum design of the tip portion.

Note that the tip portion may be bent towards the tower of the windturbine or away from the tower, in case that the rotor blade is mountedto a wind turbine which comprises a tower.

It may be advantageous that the cant angle increases by at least 20degrees, in particular by at least 40 degrees, from the tip base to thetip.

An advantageous maximum for the cant angle may be at 80 degrees.However, note that in principle also cant angles exceeding 80 degrees oreven exceeding 90 degrees are possible.

In another advantageous embodiment, the trailing edge sweep angle and/orthe cant angle increases monotonically from the tip base to the tip ofthe rotor blade.

In another advantageous embodiment, the leading edge sweep angleincreases monotonically from the tip base to the tip of the rotor blade.

Thus, note that the sweep angles and/or the cant angle may monotonicallyincrease in the region from the tip base to the tip. However, they donot necessarily have to exhibit this monotonic behavior. According tothe specific design of the rotor blade as a whole and its designatedapplication, a non-monotonic design of the sweep angles and/or the cantangle may be advantageous, too.

In another advantageous embodiment, at least one segment of the trailingedge of the tip portion is shaped as a straight line and/or at least onesegment of the leading edge of the tip portion is shaped as a straightline.

It may for example be beneficial from a manufacturing point of view toinclude one or more substantially straight segments of the trailing edgeor the leading edge in order to facilitate manufacturing of the tipportion. Such a segment is also referred to as a sub-segment of thetrailing edge or of the leading edge, respectively.

In another advantageous embodiment, the length of the tip portion, asprojected on the rotor blade plane and measured along the pitch axis, isbetween 0.5% and 10%, in particular between 1% and 5%, of the length ofthe entire rotor blade from the root to the tip as projected on therotor blade plane and measured along the pitch axis.

In other words, the tip portion comprises the part of the rotor bladewhich is the outermost part in a radial direction and comprises 0.5% to10% of the total radial length of the rotor blade. Exemplarily, the tipportion is defined as the outermost 0.5 meters to 3 meters of the rotorblade.

Note that the given relative and absolute values refer to a distancealong the pitch axis which is not necessarily equal to a distance alongthe span line of the rotor blade.

In another advantageous embodiment, the rotor blade is shaped such thatthe tip is separated from a tangential plane, wherein the tangentialplane is defined to be perpendicular to the pitch axis and tangential tothe tip portion.

Figuratively speaking, the tangential plane is tangential to theoutermost point of the rotor blade, wherein outermost is defined asfurthest away from the root and measured along the pitch axis.

Particularly in a case where the tip portion is heavily curved and/or ina case where the blade body comprises a significant sweep, there may bea separation or off-set between the tip and the tangential plane.However, advantageously, this separation between the tip and thetangential plane is below 1% of the total length of the rotor blade,defined by the distance from the tip to the root along the pitch axis.In absolute values, the separation may be in a range between 1centimeter and 100 centimeters.

It shall be noted that in the context of the whole application, the tipof the rotor blade may also be characterized as the point where theleading edge meets the trailing edge.

In another advantageous embodiment, the rotor blade deforms under windloads such that the trailing edge sweep angle and/or the chord tiltangle and/or the cant angle changes at least about 5%, in particularabout 15%, compared to an unloaded state of the rotor blade.

It shall be stressed that in the context of this application, featuressuch as angles, dimensions or positioning relative to each other relateto a rotor blade in an unloaded state, i.e. in a state without windloads acting on the rotor blade. These features may change if wind loadsact on the rotor blade. In particular, the tip portion is susceptible toa considerable structural deformation under wind loads. Thus, the givenvalues for the mentioned feature may change if wind loads are acting onthe rotor blade.

In another advantageous embodiment, the tip portion is added to theblade body as a retrofit.

On the one hand, a retrofit refers to a part, in this case the tipportion, which is added on an existing rotor blade. For instance, a windturbine may already be in operation and at a given moment the tipportion is added to the existing rotor blade. This may be done withoutdisassembling the wind turbine, in particular without dismounting therotor blade from the hub of the wind turbine. However, it may also bepossible to dismount the rotor blade, attach the retrofit tip portionand subsequently remount the rotor blade to the hub of the wind turbine.

On the other hand, retrofitting shall relate to the fact that the rotorblade as such is not manufactured as a single piece but is manufacturedwith a conventional tip portion, or no tip portion, and subsequently theinventive tip portion is added to the blade body during themanufacturing process.

In another advantageous embodiment, the rotor blade is manufactured as asingle piece, i.e. unitarily.

An advantage of manufacturing the rotor blade unitarily is a goodmechanical stability at the tip base. However, it has to be kept in mindthat transportation of the manufactured rotor blade has to beconsidered, too, which might be a challenge, in particular if the tipportion is heavily curved and/or the tip portion has a considerableextension.

Finally, in another advantageous embodiment, the trailing edge and thestreamline of an airflow which flows from the leading edge across therotor blade comprise an angle which is smaller than 80 degrees, inparticular smaller than 50 degrees at the tip portion of the rotorblade.

It has been found that an angle which is smaller than 80 degrees betweenthe trailing edge and the streamline may reduce aerodynamic noise whichis generated by the rotor blade under operation. This is due todirectivity effects on the sound pressure level. A separation of theairflow may also be the cause of undesired aerodynamic noise. One effectof the curved and twisted trailing edge of the tip portion is that thestreamline of the airflow is guided such that the angle between thestreamline and the trailing edge is reduced compared to a conventionaltip portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are now described, by way of example only,with reference to the accompanying drawings, of which:

FIG. 1 shows a wind turbine;

FIG. 2 shows a rotor blade in a top view onto the rotor blade plane;

FIG. 3 shows a tip portion of a rotor blade in a top view onto the rotorblade plane;

FIG. 4 shows a rotor blade with a swept blade body in a top view ontothe rotor blade plane;

FIG. 5 shows a perspective view of a rotor blade comprising a cantangle;

FIG. 6 shows a conventional tip portion and a raked tip portion of arotor blade;

FIG. 7 shows a raked tip portion added as a retrofit to a conventionaltip portion of a rotor blade;

FIG. 8 shows a tip portion of a rotor blade in a top view onto the rotorblade plane;

FIG. 9 shows the chord lengths of the tip portion shown in FIG. 8;

FIG. 10 shows the chord tilt angles of the tip portion shown in FIG. 8;

FIGS. 11 a-f shows the cross-sections of the tip portion shown in FIG. 8at specific radial positions;

FIG. 12 shows the deflection of the streamline of an airflow which flowsfrom the leading edge across the rotor blade;

FIG. 13 shows a tip portion of a rotor blade in a top view onto thereference plane; and

FIG. 14 shows another embodiment of a rotor blade.

The illustration in the drawings is schematically. It should furthermorebe noted that numerals which refer to similar features or elements arereferred to with the same numeral throughout the drawings.

DETAILED DESCRIPTION

In FIG. 1, a wind turbine 10, erected on a ground 14, is shown. Notethat, in general, wind turbines may be installed onshore or offshore.The wind turbine 10 in FIG. 1 comprises a tower 11 and a nacelle 12,wherein the nacelle 12 is rotatable mounted to the tower 11 by means ofa yaw bearing. The wind turbine 10 furthermore comprises a hub 13. Thehub 13 is rotatable mounted to the nacelle 12 by means of a bearing. Thehub 13 is connected to a rotor which, inside the nacelle 12, isconnected to a rotor part of a generator. The generator is arranged andprepared to convert rotational movement from the hub 13 into electricalenergy. Three rotor blades 20 (two of them are shown in FIG. 1) areconnected to the hub 13. The rotor blades 20 are arranged and preparedfor being pitched about a respective pitch axis 23. The pitch axis 23coincides with a longitudinal axis of the rotor blade 20. A controlmechanism adjusts a pitch angle such that an optimum power is generatedby the wind turbine 10 depending, amongst other parameters, on theactual wind speed and wind direction. In general, the wind turbine maybe a direct drive wind turbine, meaning that the rotor is directlyconnected to the generator. Alternatively, the wind turbine may also bea geared wind turbine. In that case a gearbox may be present between thehub 13 and the generator.

FIG. 2 shows a rotor blade 20 in a top view onto a rotor blade plane andonto a suction side 22 of the rotor blade 20. The rotor blade 20 isdivided into three areas. A first area is referred to as the rootportion 21 comprising a root 41. Advantageously, the root 41 comprises ashape of a circular cylinder. The root 41 is arranged and prepared forbeing mounted to the hub 13 of the wind turbine 10. A pitch axis 23extends through the center of the circular root 41. The extension of theroot portion 47 may exemplarily comprise 5% compared to a totalextension 46 of the rotor blade 20 from the root 41 to a tip 33 of therotor blade 20.

Adjacent to the root portion 21 is the blade body 22 of the rotor blade20. The blade body 22 is also referred to as an airfoil section of therotor blade 20. The blade body 22 is configured such that it is mainlybased on aerodynamical considerations. Compared to the blade body 22,the design of the root portion 21 is mainly driven by structuralconsiderations. The blade body 22 comprises a leading edge 24 and atrailing edge 25. The leading edge 24 may, for instance, comprise asubstantially cylindrical shape, while the trailing edge 25 may, forinstance, have a substantially sharp edge. A chord is defined at eachradial position of the rotor blade 20 as the shortest straight linebetween the leading edge 24 and the trailing edge 25. A span line orcenterline may be defined as halfway between the leading edge 24 and thetrailing edge 25.

The area of the blade body 22, where the chord is maximum, is referredto as the shoulder 42 of the rotor blade 20. Furthermore, a suction side26 and a pressure side 27 are assigned to the rotor blade 20.

The third area of the rotor blade 20 is referred to as the tip portion30 of the rotor blade 20. The tip portion 30 joins the blade body 22 atthe tip base 31. Note that in general the tip base 31 is an imaginary orvirtual area of the rotor blade 20. In particular, the tip base 31 mayalso be understood as the plane being perpendicular to the pitch axis 23at the specific radial position of the pitch axis 23 where the trailingedge 25 starts to describe a concave curvature and/or the leading edgestarts to describe a convex curvature, as viewed from the suction side24 of the rotor blade 20. The chord at the tip base 31 is referred to asthe tip base chord 32.

To give an idea of the respective dimensions of a rotor blade, anexemplary rotor blade 20 may have a total length 46 characterized by thedistance between the root 41 and the tip 33 of 70 m (meters), while theroot portion 21 only measures 2 m and the tip portion 30 measures 3 m,the latter characterized by the distance 45 between the tip 33 and thetip base 31.

FIG. 3 shows a tip portion 30 of a rotor blade 20 in a top view onto therotor blade plane and onto a suction side 22 of the rotor blade 20. Therotor blade 20 comprises a straight blade body 22 and the pitch axis 23intersects the tip base chord 32 halfway. Note that the span line 43 ofthe rotor blade 20 comprises a curved shape in the region of the tipportion 30, following the shape of the tip portion 30. The tip base 31comprises the cross-section of the rotor blade 20 at the specific radialposition where the tip portion 30 joins the blade body 22.

The tip portion 30 comprises a leading edge 24 and a trailing edge 25.Furthermore, it comprises a tip 33 which is slightly separated from atangential plane 29 which is defined to be the plane which isperpendicular to the pitch axis 23 and tangential to the tip portion 30.Exemplarily, the distance between the tip base 31 and the tangentialplane 29 is 2.5 m, while the distance between the tangential plane 29and the tip 33 is 1.5 cm. A trailing edge sweep angle 34 determined atthe tip base 31 can be given for every point on the trailing edge 25 ofthe tip portion 30. The trailing edge sweep angle 34 comprises 0 degreeat the tip base 31, as it is just measured at the tip base 31. Itincreases monotonically until the tip 33. The trailing edge sweep angle34 at the tip 33 comprises exemplarily 20 degrees.

The leading edge sweep angle 35 is determined likewise. Again, theleading edge sweep angle 35 at the tip base is 0 degree while theleading edge sweep angle 35 at the tip 33 is 36 degrees. Note that theleading edge sweep angle 35 increases monotonically from the tip base 31to the tangential plane 29 and slightly decreases between the tangentialplane 29 and the tip 33.

In FIG. 4, a rotor blade 20 with a swept blade body 22 in a top viewonto the rotor blade plane and onto a suction side 22 of the rotor blade20 is shown. Similar to a rotor blade with a straight blade body, therotor blade 20 with a swept blade body 22 can be divided into a rootportion 21, the blade body 22 and a tip portion 30. In FIG. 4, both aconventional tip portion 301 and a tip portion 30 according to oneembodiment of the invention are illustrated. The trailing edge sweepangle 341 at the tip base 31 comprises 35 degrees and the trailing edgesweep angle 34 at the tip 33 comprises 60 degrees. Thus, the trailingedge sweep angle 34 increases by 25 degrees between the tip base 31 andthe tip 33. Likewise, the leading edge sweep angle increases as wellbetween the tip base 31 and the tip 33 which for sake of clarity is notshown in FIG. 4.

FIG. 5 shows a rotor blade 20 comprising a cant angle 37 in aperspective view. The rotor blade 20 can be viewed from the leading edge24. The tip base is defined by the plane which comprises the lines I-I′,II-II′ and the tip base chord 32. FIG. 5 shows an advantageousembodiment of the invention of a rotor blade 20 with a tip portion 30which comprises a cant angle 37 of approximately 50 degrees. In otherwords, the tip portion 30 is curved out of the rotor blade plane, in theexample of FIG. 5 towards the pressure side 27 of the rotor blade 20.

It has to be noted that the tip portion 30 additionally comprises atrailing edge which is curved towards the trailing edge of the bladebody 22 and a trailing edge of the tip portion 30 that is twisted.However, the curvature of the trailing edge of the tip portion 30 andthe twist of the trailing edge of the tip portion 30 is not visible inthe perspective view of FIG. 6. A tip portion with a cant angle 37 isalso referred to as a winglet. An advantage of a tip portion 30 withsuch a cant angle 37 is a further improvement of the efficiency and afurther noise reduction of the rotor blade 20 compared to a rotor bladewithout a winglet.

FIGS. 6 and 7 show two comparisons of exemplary raked tip portions 30according to the invention and conventional tip portions 301.

In FIG. 6, the raked tip portion 30 comprises a similar length as theconventional tip portion 301, wherein the length is defined as thedistance between a first imaginary line A, which can be identified withthe tip base, and a second imaginary line B, which is the projection ofthe tip 33 onto the pitch axis 23.

In FIG. 7, the raked tip portion 30 represents an extension or an add-onto the conventional tip portion 301. While the length of theconventional tip portion 301 extends from the imaginary line A to theimaginary line B1 and is similar to the length of the conventional tipportion 301 of FIG. 6, the length of the raked tip portion 30 in FIG. 8extends from line A to line B2 and thus is greater than the length ofthe conventional tip portion 301. An embodiment as represented in FIG. 7may advantageously be used when retrofitting an existing rotor blade.

FIG. 8 shows a tip portion 30 of a rotor blade in a top view onto therotor blade plane. Chords 44 at the specific radial positions A-A′,B-B′, C-C′, D-D′, E-E′ and F-F′ are shown. The chord at the positionA-A′ is also referred to as the tip base chord 32. At the tip 33, thechord length approaches zero, thus no chord can be shown.

FIGS. 9 and 10 show the radial chord length 52 and the radial chord tiltangle 36 of the rotor blade shown in FIG. 8.

Specifically, FIG. 9 shows the chord length 52 with regard to a radialdistance from the root of the rotor blade. It can be seen that the chordlength 52 slightly decreases in the radial positions between A and E.Between the radial positions E and G, the chord length 52 decreasessignificantly more.

As can be seen in FIG. 10, the significant decrease of the chord lengthbetween the radial positions E and G is accompanied by a sharp increaseof the chord tilt angle 36 in this radial section. Note that the radialposition E, where the slope of the chord tilt angle 36 equals zero, isdenoted also as toe.

FIGS. 11 a to 11 f show the airfoil profiles, i.e. the cross-sectionsperpendicular to the pitch axis, at the radial positions A to F. Thecross-section of the radial position G is not shown as the chord lengthat the radial position G approaches zero. The dimensions of thecross-sections in FIGS. 11 a to 11 f are normalized to the cross-sectionat the tip base, i.e. at the radial position A.

In FIG. 11 a, no chord tilt angle is shown, as it equals zero at the tipbase. For radial positions extending away from the tip base and towardsthe tip, the chord tilt angle increases, as it is shown for the sectionbetween the radial position A and the radial position E in FIG. 10.Additionally, the chord tilt angle 36 is negative in this section, whichcorresponds to an inclination of the chords 44 towards a suction side 26of the blade body, as can be seen in FIGS. 11 b to 11 e. At the radialposition E, i.e. the toe, the inclination of the chord 44 towards thesuction side 26 is maximal. Going further towards the tip, theinclination first decreases and subsequently flips to the pressure side27 of the blade body. It can be seen in FIG. 11 f that the inclinationof the chord 44 at the radial position F is considerable. In particular,an angle of attack for a designed rotor speed of zero degree may berealized at the radial position F.

FIG. 12 shows a tip portion 30 of a rotor blade 20 with a raked tipportion 30, wherein the tip portion 30 is curved towards the trailingedge 25 of the blade body 22. It can be seen how the streamlines 61 ofan airflow which flows from the leading edge 24 across the rotor bladeis deflected by the tip portion 30. In a region close to the tip base,where the leading edge 24 and the trailing edge 25 are substantiallyperpendicular to the streamline 61, the streamline 61 is hardlydeflected by the rotor blade 20. However, due to the curvature of thetip portion 30, a deflection of the streamlines 61 is visible. Thus,there results an angle 62 between the trailing edge 25 and thestreamline 61 which is smaller than 90 degrees. It can also be seen thatthe angle 62 is different at different positions along the trailing edge25 of tip portion 30. A consequence of the reduced angle 62 is areduction of aerodynamic noise generated by the rotor blade 20.

In FIG. 13, another way of illustrating the twist of the trailing edge25 is shown. More specifically, a top view of the tip portion 30 ontothe reference plane is shown, wherein the reference plane is defined asthe plane comprising the pitch axis 23 and being perpendicular to thetip base chord. In other words, FIG. 13 shows a top view onto thetrailing edge 25. If the trailing edge 25 were not twisted at all, thetrailing edge 25 would just be a straight line if projected onto thereference plane. In FIG. 13, however, the trailing edge 25 is twistedwhich is manifested by the curved line of the trailing edge 25. It canbe seen that in the example shown in FIG. 13, the trailing edge 25 isfirst twisted towards the suction side 26, before being twisted towardsthe pressure side 27. Note that no chords are shown in FIG. 13 as chordsextend into, or out of, the plane which is illustrated in FIG. 13.

Finally, FIG. 14 illustrates the definition of the rotor blade plane byshowing an exemplary rotor blade 20, where the blade body 22 is curvedtowards the suction side 26. As the pitch axis 23 and the tip base chord32 do not intersect, the rotor blade plane is defined by the line 231,which is parallel to the pitch axis 23, and the tip base chord 32. Theline 231 intersects the tip base chord 32. Note that in the example ofFIG. 14, the tip 33 of the rotor blade 20 is swept out of the rotorblade plane, thus comprising a cant angle.

1. A rotor blade of a wind turbine, comprising a blade body extendingbetween a root portion of the rotor blade and a tip portion of the rotorblade, a trailing edge and a leading edge, wherein the rotor blade isarranged and prepared for being mounted to a hub of the wind turbine andfor being pitched about a pitch axis, and wherein a rotor blade plane isdefined by the plane comprising the chord at the tip base and a linewhich is parallel to the pitch axis, wherein the tip base is the part ofthe rotor blade at which the tip portion joins the blade body, whereinthe trailing edge of the tip portion has a curved shape as projected onthe rotor blade plane such that a trailing edge sweep angle increasesfrom the tip base to the tip of the rotor blade, and wherein orientationof the chords with reference to the chord at the tip base changesbetween the tip base and the tip such that a chord tilt angle changesbetween the tip base and the tip.
 2. The rotor blade according to claim1, wherein the trailing edge sweep angle increases by at least 2 degreesfrom the tip base to the tip.
 3. The rotor blade according to claim 1,wherein the trailing edge sweep angle increases by at most 40 degreesfrom the tip base to the tip.
 4. The rotor blade according to claim 1,wherein the chord tilt angle varies by at least 2 degrees between thetip base and the tip.
 5. The rotor blade according to claim 1, whereinthe chord tilt angle varies by at most 30 degrees between the tip baseand the tip.
 6. The rotor blade according to claim 1, wherein in a firstsection of the tip portion the trailing edge is twisted towards thesuction side of the blade body, thus leading to negative chord tiltangles and/or in a second section of the tip portion the trailing edgeis twisted towards the pressure side of the blade body, thus leading topositive chord tilt angles.
 7. The rotor blade according to claim 6,wherein the first section is adjacent to the tip base and the secondsection is adjacent to the tip.
 8. The rotor blade according to claim 6,wherein the rate of decrease of the chord length of the tip portion isgreater in the second section than in the first section.
 9. The rotorblade according to claim 1, wherein the leading edge of the tip portionhas a curved shape as projected on the rotor blade plane such that aleading edge sweep angle increases from the tip base to the tip.
 10. Therotor blade according to claim 9, wherein the leading edge sweep angleincreases by at least 10 degrees from the tip base to the tip.
 11. Therotor blade according to claim 9, wherein the leading edge sweep angleincreases by at most 80 degrees, in particular by at most 60 degrees,from the tip base to the tip.
 12. The rotor blade according to claim 1,wherein the tip portion is curved towards the trailing edge of the bladebody.
 13. The rotor blade according to claim 1, wherein, as viewed fromthe suction side of the rotor blade, the trailing edge of the tipportion is concavely shaped and the leading edge of the tip portion isconvexly shaped.
 14. The rotor blade according to claim 1, wherein thetrailing edge of the tip portion as projected on a reference plane,wherein the reference plane is defined as the plane comprising the pitchaxis and being perpendicular to the chord at the tip base, has a curvedshape such that a cant angle increases from the tip base to the tip. 15.The rotor blade according to claim 14, wherein the cant angle increasesby at least 20 degrees from the tip base to the tip.
 16. The rotor bladeaccording to claim 14, wherein the trailing edge sweep angle and/or thecant angle increases monotonically from the tip base to the tip.
 17. Therotor blade according to claim 1, wherein at least one segment of thetrailing edge of the tip portion is shaped as a straight line and/or atleast one segment of the leading edge of the tip portion is shaped as astraight line.
 18. The rotor blade according to claim 1, wherein thelength of the tip portion, as projected on the rotor blade plane andmeasured along the pitch axis, is between 0.5% and 10%, of the length ofthe entire rotor blade as projected on the rotor blade plane andmeasured along the pitch axis.
 19. The rotor blade according to claim 1,wherein the rotor blade is shaped such that the tip is separated from atangential plane which is defined to be perpendicular to the pitch axisand tangential to the tip portion.
 20. The rotor blade according toclaim 14, wherein the rotor blade deforms under wind loads such that thetrailing edge sweep angle and/or the chord tilt angle and/or the cantangle changes at least about 5% compared to an unloaded state of therotor blade.
 21. The rotor blade according to claim 1, wherein the tipportion is added to the blade body as a retrofit.
 22. The rotor bladeaccording to claim 1, wherein the rotor blade is manufactured as asingle piece.
 23. The rotor blade according to claim 1, wherein thetrailing edge and the streamline of an airflow which flows from theleading edge across the rotor blade comprise an angle which is smallerthan 80 degrees at the tip portion.